In a conventional electrophotographic image forming apparatus, after transferring a toner image that is formed on an image bearing member or photosensitive drum onto a sheet substrate, an electric charge which still remains on the photosensitive drum should be erased in preparation for a subsequent image forming. For this purpose, there has been proposed an erasing device in which a flexible sheet made of electrically conductive material is mounted in the vicinity of the photosensitive member in contact at one plain surface thereof. With the erasing device, applying a certain voltage to the flexible sheet will erase the residual electric charge from the photosensitive drum.
Generally, the photosensitive drum has a length of about 300 to 400 mm. Therefore, to arrange the flexible sheet along the entire length of the photosensitive drum and bring the same into close contact therewith, a support member that has a considerable rigidity should be disposed in parallel to the photosensitive drum for supporting the flexible sheet. This rigid support member must be bulky and therefore occupies a larger installation space in the vicinity of the photosensitive drum, which in turn forces other devices arrange around the photosensitive drum to be smaller.
Unexamined Japanese Utility-Model Publication No. 61-112,366 discloses an erasing device that includes an erasing blade or erasing plate made of electrically conductive rubber. The erasing plate is disposed in the vicinity of the photosensitive drum while a longitudinal edge thereof is kept in contact with the outer periphery of the photosensitive drum for erasing the residual charge from the photosensitive drum.
For the erasing device that employs such erasing member, it is preferable that the erasing member is arranged so that it contacts only at the longitudinal edge thereof whether the erasing plate is thick or thin.
This is because keeping a major surface of the erasing sheet member in face-to-face contact with the photosensitive drum rather than the line contact tends to lead a variation of position where an electric discharge occurs between the erasing plate and the photosensitive drum due to the application of a certain voltage with the erasing plate, which will fail to erase the residual charge to a necessary extent depending upon the location.
Also, biasing the erasing plate to the photosensitive drum will lead a permanent set of the erasing member at a contact portion thereof and then increase a contact area of between the erasing member and the photosensitive drum, which results in an erasing defect on the photosensitive drum.
Further, for the contact-type erasing device as described above, it can be considered that the erasing is a sort of discharging in which an electric charge leaks from erasing device to the photosensitive drum. Therefore, keeping the erasing ability of the erasing device constant will lead an over-erasion of the photosensitive drum, which results in an adverse charge memory corresponding to the reproduced image.